Guanosine 5'-monophosphate (GMP) synthetase (GuaA) catalyzes the last step of GMP synthesis in the purine nucleotide biosynthetic pathway. This enzyme catalyzes a reaction in which xanthine 5'-monophosphate (XMP) is converted to GMP in the presence of Gln and ATP through an adenyl-XMP intermediate. A structure of an XMP-bound form of GuaA from the domain Bacteria has not yet been determined. In this study, the crystal structure of an XMP-bound form of GuaA from the thermophilic bacterium Thermus thermophilus HB8 (TtGuaA) was determined at a resolution of 2.20 Å and that of an apo form of TtGuaA was determined at 2.10 Å resolution. TtGuaA forms a homodimer, and the monomer is composed of three domains, which is a typical structure for GuaA. Disordered regions in the crystal structure were obtained from the AlphaFold2-predicted model structure, and a model with substrates (Gln, XMP and ATP) was constructed for molecular-dynamics (MD) simulations. The structural fluctuations of the TtGuaA dimer as well as the interactions between the active-site residues were analyzed by MD simulations.

A method of treating cancer through use of guanosine 3',5'-cyclic monophosphate (cyclic GMP). Cyclic GMP decreases the number of human breast cancer and prostate adenocarcinoma as well as small-cell and squamous lung cells in culture by 30% (1 μM), 84% (1 mM), 31% (1 μM), and 30% (1 μM), respectively. Cyclic GMP decreases DNA synthesis in human pancreatic, breast, and prostate adenocarcinomas as well as small-cell and squamous cell carcinomas of the lung at its 1 μM concentration by 51%, 54%, 56%, 50% and 52%, respectively. Cyclic GMP when infused for one week decreases the tumor volume of human pancreatic adenocarcinomas in athymic mice 95% compared to untreated animals with human pancreatic adenocarcinomas.